Your boiler has stopped working because it cannot properly detect the flame it needs to stay lit safely.

Burner blocked - Internal error (Check ionisation electrode and leads/flue gas system)

What causes this fault?

This fault occurs when the boiler's internal sensors fail to confirm that a steady flame has been established, causing the system to shut down for safety. The most common reason is a worn or carbon-coated detection probe that can no longer 'see' the flame, though it can also be caused by a blockage in the air supply or chimney preventing clean combustion.